Present invention concerns a process and design to ensure the safety of a liquefied gas underground storage.
Usually the storage involves several galleries of specific size and lay-out and located at calculated depth according to the stored product so that the pressure of water in the surrounding rock is slightly greater than the liquefied gas vapour pressure at the ground temperature which means that ground water can flow towards the storage and prevent the liquefied gas from flowing outwards so that the storage is tight. Inside a storage gallery the ground water is dripping along the walls towards the floor and accumulates in the low points. Above the water is the liquid phase of the liquefied gas and in all the upper volume of the storage is in the gas phase.
Various piping connections provide the necessary links between storage and surface to operate the storage, such as, filling and pumping out the product, pumping out the seepage water, level and temperature indications, alarms.
With regard to danger aspect, two cases may be considered. Liquefied natural gas (mainly methane) is lighter than air and leaks are not too dangerous considering that they very quickly dilute and disappear towards the upper atmosphere. The other liquefied gases such as propane or butane are heavier than air and are very dangerous considering that leaks create layers on the ground flowing and diluting very slowly in windless atmosphere.
Concerning the danger of the leaks that may happen on a piping connection between storage and surface the vent line linking directly the gas phase to the surface must be considered separately: any break on the vent may result in emptying completely the storage quickly and continuously because the whole storage surrounding rock provides a tremendous heat source to boil the stored product after a partial pressure drop and correlated equilibrium temperature drop. The other liquid connections are potentially less dangerous considering that the liquid column in the leaking vertical pipe partly balances the storage pressure and that the only boiling takes place in the upper part of the leaking pipe which represents a very limited heat source compared to the whole storage.
Usually the vent line is a vertical rising fixed pipe connecting directly the gas phase to the surface and which cannot be isolated. The design according to the invention solves that disadvantage by the removal of the vent connection in case of danger straight inside the storage.
Usually the connections from storage to surface involve heavy mechanical, electrical or instrument equipment located inside the storage. Their maintenance or repair may require frequent operations, potentially dangerous, needing heavy equipment. The process according to the invention solves that disadvantage by removing all heavy equipment out of the storage and moving their function to ground level.
Usually each connection between storage and surface is devoted to one function only, resulting in the impossibility of replacing one with another, which can lead to potentially dangerous operating situations in case of temporary or definitive failure. The process according to the invention solves that problem by the standardization of all the connections except the vent line which permits replacement of one by another and preserving the safety of operations.
Usually the connections between storage and the surface are fitted with safety valves and sometimes some can be better isolated at the level of the storage by means of a water plug balancing the storage pressure; but the creation of these plugs needs manual operation and water injection from the surface at the time they are decided which may be difficult or even impossible if the area is already dangerous or equipment already destroyed. The design according to the invention solves the problem as it gives the possibility to isolate all connections from the storage including the vent line with water plugs which are created automatically without manual operation or water injection because the water for the plugs is already present in reserves inside the storage.
Usually the underground storage is protected against overfilling which is the main risk as to the possibility of overpressure mainly during reception of large refrigerated cargoes at a large flow rate. Nevertheless the protections are based upon alarms and safety set points given by level instruments which do not measure always the true storage level and which may drift with time. The design according to the invention solves this problem as it is related to the true storage level when it detects the maximum allowed filling level and that the set point cannot break down.